Tests: a precompiled header for the test executable

[revarbat]

All 35 test files include gtest, and 28 include evaluator.hpp. Header parsing is 40–52% of a test file's compile (clang, measured by compiling each file against a headers-only copy of itself). This adds one target_precompile_headers for oscad_eval_tests, covering gtest, evaluator.hpp, the parser API and the shared std headers.

Local measurement (clang, macOS, the test executable's objects rebuilt at -j 4):

	Wall	CPU
Before	26.6, 26.7 s	90.5, 91.9 s
After	24.8, 26.2 s	70.8, 74.8 s

That's ~20% less CPU but only 2–7% less wall time: the PCH compiles before any test file can start, and the end of the build is dominated by the largest test files.

CI result: small but consistent on Windows, three runs paired against #222's three runs (/MP only):

CI run	Windows, /MP	Windows, /MP + PCH	Change
1st	511 s	460 s	−10%
2nd	499 s	462 s	−7%
3rd	353 s	322 s	−9%

That's ~40 s off each Windows CI build. Linux and macOS are within their run-to-run noise. Release wheels don't build the tests, so they're unaffected.

The trade-off: a test file that forgets one of the precompiled includes still compiles here. That only affects test code.

🤖 Generated with Claude Code